Allyl-Functionalized Anchors

The cleavage of the double bond with ozone was first reported in 1971 by Frechet for the release of aldehydes from the solid-phase carrier. These anchors are described under the heading Miscellaneous anchors cleaved by oxidation . 

4-Bromocrotonic acid Cleavage 0.1 eq (Ph,P)4Pd + nucleophile (HOBt, morpholine.) in DMF Product after cleavage Carboxylic acid 

Diolefinic linkers Cleavage Product after cleavage  

Linker 1 is immobilized through the carboxy function. Linker 2 is generated on solid support by a Wittig reaction. A second alkene is generated during or at the end of the synthesis to give the second olefin for metathesis. 

In principle, all reactions that do not interact with olefinic structures. 

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A further orthogonal strategy compatible with peptide and many other organic synthesis is offered by allyl-functionalized anchors. The mechanism of release is depicted in Figure 5.8, and involves the use an intermediate Pd-complex with the double bond [267,268], Recent developments use Pd-catalyzed metathesis techniques to cleave unsaturated anchors in inter- and intramolecular reactions. An unsaturated tin-anchor has been used to achieve cleavage with an intramolecular Suzuki reaction. 

Hydrosilylation of allyl chloride with HSiCl3 leads to Cl(CH2)3SiCl3 that can be used for many surface modifications after substitution of the chlorides by suitable functional groups [5], For instance it can be used to anchor soluble hydroformylation catalysts to a silica surface (Figure 18.3). Many examples have been reported and we present only one example of a selective Xantphos type ligand [6],... 

A major limitation to the Sharpless-Katsuki epoxidation is that its utility is largely confined to oxidation of allylic alcohols. Homoallylic alcohols are oxidized less cleanly and the oxidation of simple olefins shows little enantio-selectivity. This is presumably because the stereochemical control depends on anchoring the substrate to a particular site on the metal by means of an auxiliary coordinating function. 

The enzymatic deprotection principle has also been applied in solid phase synthesis to create an anchoring structure that contains two cleavable positions an allylic ester on the one hand and a hydrophilic ester on the other hand. The release of j ptide using a palladium(O) catalyst delivers the peptide that is still bound to the hydrophilic structure. The carboxylic function can be liberated [48] using lipase catalysts (Fig. 32). This strategy was successfully demonstrated in the solid phase synthesis of peptides as well as O- and iV-glycopeptides [48]. 

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